Apparatus and method using a remote RF energized plasma for processing semiconductor wafers

ABSTRACT

Chemical generator and method for generating a chemical species at a point of use such as the chamber of a reactor in which a workpiece such as a semiconductor wafer is to be processed. The species is generated by creating free radicals, and combining the free radicals to form the chemical species at the point of use.

This application is a continuation of commonly-own U.S. patentapplication Ser. No. 09/225,922 filed Jan. 5, 1999, now U.S. Pat. No.6,579,805.

This invention pertains generally to the fabrication of semiconductordevices and, more particularly, to a method and apparatus for generatingimportant chemical species in the deposition, etching, cleaning, andgrowth of various materials and layers.

It is in general an object of the invention to provide a new andimproved chemical generator and method for generating chemical speciesat or near the location where they are to be used.

Another object of the invention is to provide a chemical generator andmethod of the above character which are particularly suitable forgenerating chemical species for use in the fabrication of semiconductordevices.

These and other objects are achieved in accordance with the invention byproviding a chemical generator and method for generating a chemicalspecies at or near a point of use such as the chamber of a reactor inwhich a workpiece such as a semiconductor wafer is to be processed. Thespecies is generated by creating free radicals, and combining the freeradicals to form the chemical species at or near the point of use.

FIG. 1 is a diagrammatic view of one embodiment of an in situ chemicalgenerator incorporating the invention.

FIG. 2 is an enlarged cross-sectional view taken along line 2—2 of FIG.1.

As illustrated in FIG. 1, the chemical generator includes a free radicalsource 11 which has one or more chambers in which free radicals arecreated and delivered for recombination into stable species. In theembodiment illustrated, the source has three chambers which are formedby elongated, concentric tubes 12–14. Those chambers include a firstannular chamber 16 between the outermost tube 12 and the middle tube 13,a second annular chamber 17 between middle tube 13 and the innermosttube 14, and a third chamber 18 inside the innermost tube The tubes arefabricated of a material such as ceramic, quartz or metal.

The number of tubes which are required in the generator is dependentupon the chemical species being generated and the reaction by which itis formed, with a separate chamber usually, but hot necessarily, beingprovided for each type of free radical to be used in the process.

Gases or other precursor compounds from which the free radicals areformed are introduced into the chambers from sources 21–23 or by othersuitable means. Such precursors can be in gaseous, liquid and/or solidform, or a combination thereof.

A plasma is formed within the one or more chambers to create the freeradicals, and in the embodiment illustrated, the means for generatingthe plasma includes an induction coil 26 disposed concentrically aboutthe one or more tubes, a radio frequency (RF) power generator 27connected to the coil by a matching network 28, and a Tesla coil 29 forstriking an arc to ignite the plasma. The plasma can, however, be formedby any other suitable means such as RF electrodes or microwaves.

Downstream of, or within, the tubes, the free radicals are recombined toform the desired species. In the embodiment illustrated, a recombinationmay take place in a chamber 31 which is part of a reactor 32 in which asemiconductor wafer 33 is being processed. Recombination can be promotedby any suitable means such as by cooling 36 and/or by the use of acatalyst 37.

Cooling can be effected in a number of ways, including the circulationof a coolant such as an inert gas, liquid nitrogen, liquid helium orcooled water through tubes or other suitable means in heat exchangerelationship with the reacting gases. It can also be effected by passingthe gases through an expansion nozzle to lower their temperature, or bythe use of either a permanent magnet or an electromagnet to converge andthen subsequently expand the plasma to lower its temperature.

A catalyst can be placed either in the cooling zone or downstream of it.It can, for example, be in the form of a thin film deposited on the wallof a chamber or tube through which the reacting gases pass, a gauzeplaced in the stream of gas, or a packed bed. The important thing isthat the catalyst be situated in such a way that all of the gas is ableto contact its surface and react with it.

If desired, monitoring equipment such as an optical emissionspectrometer can be provided for monitoring parameters such as speciesprofile and steam generation.

In the embodiment illustrated, the chemical generator is integrated withthe reactor, and the species produced is formed in close proximity tothe wafer being processed. That is the preferred application of thegenerator, although it can also be used in stand-alone applications aswell. It can be added to existing process reactors as well as beingconstructed as an integral part of new reactors, or as a stand-alonesystem.

The generator can be employed in a wide variety of applications forgenerating different species for use in the fabrication of semiconductordevices, some examples of which are given below.

Oxidation

Steam for use in a wet oxidation process for producing SiO₂ according tothe reactionSi+H₂O→SiO₂+H₂can be generated in accordance with the invention by admitting H₂ and O₂into one of the plasma generating chambers. The H₂ and O₂ react to formsteam in close proximity to the silicon wafer. If desired, oxygenadmitted alone or with N₂ and/or Ar can be used to produce ozone (O₃) tolower the temperature for oxidation and/or improve devicecharacteristics.

It is known that the use of NO in the oxidation of silicon with O₂ canimprove the device characteristics of a transistor by improving theinterface between silicon and silicon oxide which functions as a barrierto boron. Conventionally, NO is supplied to the reactor chamber from asource such as a cylinder, and since NO is toxic, special precautionsmust be taken to avoid leaks in the gas lines which connect the sourceto the reactor. Also, the purity of the NO gas is a significant factorin the final quality of the interface formed between the silicon and thesilicon oxide, but it is difficult to produce extremely pure NO.

With the invention, highly pure NO can be produced at the point of usethrough the reactionN₂+O₂→2NOby admitting N₂ and O₂ to one of the chambers and striking a plasma.When the plasma is struck, the N₂ and O₂ combine to form NO in closeproximity to the wafer. Thus, NO can be produced only when it is needed,and right at the point of use, thereby eliminating the need forexpensive and potentially hazardous gas lines.

NO can also be produced by other reactions such as the cracking of amolecule containing only nitrogen and oxygen, such as N₂O. The NO isproduced by admitting N₂O to the plasma chamber by itself or with O₂. Ifdesired, a gas such as Ar can be used as a carrier gas in order tofacilitate formation of the plasma.

N₂O can be cracked either by itself or with a small amount of O₂ to formNO₂, which then dissociates to NO and O₂. In rapid thermal processingchambers and diffusion furnaces where temperatures are higher than thetemperature for complete dissociation of NO₂ to NO and O₂ (620° C.), theaddition of NO₂ will assist in the oxidation of silicon for gateapplications where it has been found that nitrogen assists as a barrierfor boron diffusion. At temperature below 650° C., a catalyst can beused to promote the conversion of NO₂ to NO and O₂. If desired, nitricacid can be generated by adding water vapor or additional H₂ and O₂ inthe proper proportions.

Similarly, NH₃ and O₂ can be combined in the plasma chamber to produceNO and steam at the point of use through the reactionNH₃+O₂→NO+H₂O.

By using these two reagent gases, the efficacy of NO in the wetoxidation process can be mimicked.

It is often desired to include chlorine in an oxidation process becauseit has been found to enhance oxidation as well as gettering unwantedforeign contaminants. Using any chlorine source such as TCA or DCE,complete combustion can be achieved in the presence of O₂, yieldingHCl+H₂O+CO₂. Using chlorine alone with H₂ and O₂ will also yield HCl andH₂O.

When TCA or DCE is used in oxidation processes, it is completelyoxidized at temperatures above 700° C. to form HCl and carbon dioxide inreactions such as the following:C₂H₃Cl₃+2O₂→2CO₂+3HClC₂H₂Cl₂+2O₂→2CO₂+2HCl

The HCl is further oxidized in an equilibrium reaction:4HCl+O₂→2H₂O+Cl₂

Decomposition of various organic chlorides with oxygen at elevatedtemperatures provides chlorine and oxygen-containing reagents forsubsequent reactions in, e.g., silicon processing. Such decomposition isgenerally of the formC_(x)H_(y)Cl_(y)+xO₂→xCO₂+yHCl,where x and y are typically 2, 3 or 4.

All of the foregoing reactions can be run under either atmospheric orsubatmospheric conditions, and the products can be generated with orwithout a catalyst such as platinum.

The invention can also be employed in the cleaning of quartz tubes forfurnaces or in the selective etching or stripping of nitride orpolysilicon films from a quartz or silicon oxide layer. This isaccomplished by admitting a reactant containing fluorine and chlorinesuch as a freon gas or liquid, i.e. C_(x)H_(y)F_(z)Cl_(q), wherex=1, 2, . . .y=0, 1, . . .z=0, 1, . . .q=0, 1, . . .and the amount of fluorine is equal to or greater than the amount ofchlorine. It is also possible to use a mixture of fluorinated gases(e.g., CHF₃, CF₄, etc.) and chlorinated liquids (e.g., CHCl₃, CCL₄,etc.) in a ratio which provides effective stripping of the nitride orpolysilicon layer.

Dielectric Films

Other dielectric films can be formed from appropriate precursor gases.Polysilicon can be formed using SiH₄ and H₂, or silane alone. The silanemay be introduced downstream of the generator to avoid nucleation andparticle formation.

Silicon nitride can be formed by using NH₃ or N₂ with silane (SiH₄) orone of the higher silanes, e.g. Si₂H₆. The silane can be introduceddownstream of the generator to avoid nucleation and particle formation.

In addition to gases, the chemical generator is also capable of usingliquids and solids as starting materials, so that precursors such asTEOS can be used in the formation of conformal coatings. Ozone and TEOShave been found to be an effective mixture for the deposition of uniformlayers.

Metal and Metal Oxide Films

Metal and metal oxide films can be deposited via various precursors inaccordance with the invention. For example, Ta₂O₅ films which are usedextensively in memory devises can be formed by generating a precursorsuch as TaCl₅ via reduction of TaCl₅, followed by oxidation of the TaCl₅to form Ta₂O₅. In a more general sense, the precursor from which theTa₂O₅ is generated can be expressed as TaX_(m), where X is a halogenspecies, and m is the stoichiometric number.

Copper can be, deposited as a film or an oxide through the reactionCuCl₂+H₂→Cu+HCl,and other metals can be formed in the same way. Instead of a gaseousprecursor, a solid precursor such as Cu or another metal can also beused.

Wafer and Chamber Cleaning

With the invention, organic residue from previous process steps can beeffectively removed by using O₂ to form ozone which is quite effectivein the removal of organic contaminants. In addition, reacting H₂ with anexcess of O₂ will produce steam and O₂ as well as other oxygen radicals,all of which are effective in eliminating organic residue. Thetemperature in the chamber should be below about 700° C. if a wafer ispresent, in order to prevent oxide formation during the cleaningprocess.

Sulfuric acid, nitric acid and hydrofluoric acid for use in generalwafer cleaning are also effectively produced with the invention.Sulfuric acid (H₂SO₄) is generated by reacting either S, SO or SO₂ withH₂ and O₂ in accordance with reaction such as the following:S+2.5O₂+2H₂→H₂SO₄+H₂OSO+1.5O₂+H₂→H₂SO₄SO₂+1.5O₂+2H₂→H₂SO₄+H₂Othen quickly quenching the free radicals thus formed with or without acatalyst.

Nitric acid (HNO₃) is generated by reacting NH₃ with H₂ and O₂, or by areaction such as the following:N₂+3.5O₂+H₂→2HNO₃+H₂ONH₃+2O₂→2HNO₃+H₂O

Hydrofluoric acid is generated by co-reacting H₂ and O₂ with a compoundcontaining fluorine such as NF₃ or C_(x)H_(y)F_(z), wherex=1, 2, . . .y=0, 1, . . .z=1, 2, . . .

Mixed acids can be generated from a single precursor by reactions suchas the following:SF₆+4H₂+2O₂→H₂SO₄+6HFNH₂+H₂+1.5O₂→HNO₃+HF2NHF+H₂+3O₂→2HNO₃+2HFNF₃O+2H₂+O₂→HNO₃+3HFNF₂Cl+2H₂+1.5O₂→HNO₃+2HF+HClN₂F₄+3H₂+3O₂→2HNO₃+4HFN₂F₄+2H₂+3O₂→2HNO₃+2HFNF₃+2H₂+1.5O₂→HNO₃+3HFNF₂+1.5H₂+1.5O₂→HNO₃+2HFNF+H₂+1.5O₂→HNO₃+HFNS+1.5H₂+3.5O₂→HNO₃+H₂SO₄2N₂OF+2H₂+O₂→2HNO₃+2HFNOF₃+2H₂+O₂→HNO₃+3HFNOF+H₂+O₂→HNO₃+HFNOCl+H₂+O₂→HNO₃+HClNOBr+H₂+O₂→HNO₃+HBrNO₂Cl+2H₂+O₂→2HNO₃+HClS₂F₁O+7H₂+4O₂→H₂SO₄+10HFS₂F₂+3H₂+4O₂→H₂SO₄+2HFSF+1.5H₂+2O₂→H₂SO₄+HFSF₂+2H₂+2O₂→H₂SO₄+2HFSF₃+2.5H₂+2O₂→H₂SO₄+3HFSF₄+3H₂+2O₂→H₂SO₄+4HFSF₅+3.5H₂+2O₂→H₂SO₄+5HFSF₆+4H₂+2O₂→H₂SO₄+6HFSBrF₅+4H₂+2O₂→H₂SO₄+5HF+HBrS₂Br₂+3H₂+4O₂→2H₂SO₄+2HBrSBr₂+2H₂+2O₂→H₂SO₄+2HBrSO₂F₂+2H₂+O₂→H₂SO₄+2HFSOF₄+3H₂+1.5O₂→H₂SO₄+4HFSOF₂+2H₂+1.5O₂→H₂SO₄+2HFSOF+1.5H₂+1.5O₂→H₂SO₄+HFSO₂ClF+2H₂+O₂→H₂SO₄+HF+HClSOCl₂+2H₂+1.5O₂→H₂SO₄+2HClSOCl+1.5H₂+1.5O₂→H₂SO₄+HClSOBr₂+2H₂+1.5O₂→H₂SO₄+2HBrClSF₂Cl+2.5H₂+2O₂→H₂SO₄+2HF+HClSClF₅+4H₂+2O₂→H₂SO₄+5HF+HClSO₂Cl₂+2H₂+O₂→H₂SO₄+2HClS₂Cl+2.5H₂+4O₂→2H₂SO₄+HClSCl₂+2H₂+2O₂→H₂SO₄+2HCl

These are but a few examples of the many reactions by which mixed acidscan be generated in accordance with the invention. Including more H₂ andO₂ in the reactions will allow steam to be generated in addition to themixtures of acids.

In order to devolitize the various resultant products of the reaction ofHCl, HF, H₂SO₄ or HNO₃, either H₂O or H₂ and O₂ can be co-injected toform steam so that the solvating action of water will disperse insolution in the products. The temperature of the water must be coolenough so that a thin film of water will condense on the wafer surface.Raising the temperature of the water will evaporate the water solution,and spinning the wafer will further assist in the removal process.

Native Oxide Removal

The native oxide which is ever present when a silicon wafer is exposedto the atmosphere can be selectively eliminated by a combination of HFand steam formed by adding a fluorine source such as NF₃ or CF₄ to thereagent gases H₂ and O₂. In order for the native oxide elimination to bemost effective, the reaction chamber should be maintained at a pressurebelow one atmosphere.

Photoresist Stripping

H₂ and O₂ can also be reacted to form steam for use in the stripping ofphotoresist which is commonly used in patterning of silicon wafers inthe manufacture of integrated circuits. In addition, other componentssuch as HF, H₂SO₄ and HNO₃ which are also generated with the inventioncan be used in varying combinations with the steam to effectively removephotoresist from the wafer surface. Hard implanted photoresist as wellas residues in vias can also be removed with steam in combination withthese acids.

SO₃ for use in the stripping of organic photoresist can be generated byadding O₂ to SO₂. Similarly, as discussed above, N₂O can be converted toNO₂, a strong oxidizing agent which can also be used in the stripping ofphotoresist.

Hydrofluoric acid for use in the stripping of photoresist can begenerated in situ in accordance with any of the following reactions:CF₄+2H₂+O₂→CO₂+4HFCF₄+1.5O₂+3H₂→CO₂+4HF+H₂ONF₃+O₂+5H₂→N₂+6HF+2H₂O

It is apparent from the foregoing that a new and improved chemicalgenerator and method have been provided. While only certain presentlypreferred embodiments have been described in detail, as will be apparentto those familiar with the art, certain changes and modifications can bemade without departing from the scope of the invention as defined by thefollowing claims.

1. A method of using a remote RF plasma for processing semiconductorwafers, comprising: generating free radicals from first RF energizedplasma of a first precursor material in a first plasma generatingchamber and generating free radicals from a second RF energized plasmaof a second precursor material in a second plasma generating chamber,the second plasma generating chamber being concentrically about thefirst plasma generating chamber, the first plasma generating chamber andthe second plasma generating chamber being external of and fluidallycoupled to a reactor chamber for processing at least one semiconductorwafer.
 2. The method according to claim 1 further comprising: cleaningsaid reactor chamber through interaction of said free radicals with saidreactor chamber.
 3. The method according to claim 1 further comprising:etching material from said at least one semiconductor wafer throughinteraction of said free radicals with said at least one semiconductorwafer.
 4. The method according to claim 1 further comprising: generatingsaid RF energized plasma by coupling an RF generator to an inductioncoil disposed concentrically around said first plasma generating chamberand said second plasma generating chamber.
 5. The method according toclaim 1 further comprising: generating said RF energized plasma bycoupling an RF generator to an induction coil disposed concentricallyaround an elongated tube, wherein said elongated tube has an innercavity defining said first plasma generating chamber.
 6. The methodaccording to claim 1 further comprising: generating said RF energizedplasma by coupling an RF generator to RF electrodes disposedconcentrically about said first plasma generating chamber.
 7. The methodaccording to claim 1 further comprising: generating a chemical speciesthrough combination of said free radicals for processing said at leastone semiconductor wafer.
 8. The method according to claim 7 furthercomprising: depositing material on said at least one semiconductor waferthrough interaction of said chemical species with said at least onesemiconductor wafer.
 9. The method according to claim 7 furthercomprising: etching material from said at least one semiconductor waferthrough interaction of said chemical species with said at least onesemiconductor wafer.